1. Field of the Invention
The present disclosure relates generally to carbon nanostructures, methods of covalently functionalizing carbon nanostructures, and methods of separating and isolating covalently functionalized carbon nanostructures.
2. Background
Carbon nanotubes (CNTs) are self-assembling nanostructures comprised of graphite sheets rolled up into cylinders (Iijima, Nature 354:56-58 (1991)). Such nanostructures are termed single-walled CNTs (SWNTs) if they are comprised of a single cylindrical tube (Iijima et al., Nature 363:603-605 (1993); Bethune et al., Nature 363:605-607 (1993)). CNTs comprising two or more concentric tubes are termed double-walled CNTs (DWNTs) and multi-walled CNTs (MWNTs), respectively. Regarding SWNTs, the diameter of these species can typically range from 0.4 nm to ca. 3 nm, and the length from ca. 10 nm to centimeters.
CNTs possess outstanding structural, mechanical, and electronic properties due to the unique combination of their dimension, structure, and topology. Thus, CNTs have found use in a wide variety of applications including conductive and high-strength composites, electrode materials for high capacity batteries, efficient field emission displays and radiation sources, and functional nanoscale devices (Baughman et al., Science 297:787-792 (2002)).
Separating CNTs by structure is a challenge. Within a small diameter range (0.4-2 nm), a SWNT can have over 300 possible chiral structures, each uniquely indexed by a pair of integers (n,m). Two SWNTs, e.g., (10,10) vs. (11,9), may differ in diameter by less than 0.01 nm, amounting to a mere 0.15% difference in density. Distinguishing this minute structural difference is currently beyond the reach of even the most successful separation methods developed (Hersam, Nature Nanotechnology 3:387-394 (2008)) which rely on surface properties (Peng et al., Nature Nanotechnology 2:361-365 (2007): Zheng and Semke, J. Am. Chem. Soc. 129:6084-6085 (2007)) or density discrimination (Arnold et al., Nature Nanotechnology 1:60-65 (2006)).
There exists a need to develop methods to synthesize a single type of CNT displaying only semiconductor or metallic properties. While some progress is being made in this field, there is still no perfect method. In response to synthetic limitations, several chemical separations methods have been developed to purify CNT samples of different chiralities or electronic character from bulk samples of mixed species. DNA wrapped CNTs can be chirally resolved and separated by modifying the DNA nucleotide sequence and passing the sample through an ion exchange column (Tu et al., Nature 460:250-253 (2009)). This approach produces perhaps the purest samples of semiconductors. However, the procedure is optimized for semiconductor CNTs and fails to achieve a similar level of separation for metallic CNTs. Additionally, the usage of DNA is expensive and yields are limited to between 0.1 to 0.8 μg of chirally resolved semiconducting CNTS for every 100 μg of raw CNT sample. Ultracentrifugation of surfactant encapsulated CNTs has also been shown capable of sorting CNTs by buoyant density, which allows for the enrichment of metallic types (Arnold et al., Nature Nanotechnology 1:60-65 (2006); Green and Hersam, Mater. Today 10:59-60 (2007)). The technique has shown reasonable yields to the μg level and scalability is predicted to increase with the use of industrial size centrifuges which could increase processing scale to gram levels. However, the use of surfactants and ultracentrifugation ultimately limit scalability and requires excessive sonication which can damage CNTs and diminish their attractive properties.
In addition to these physical separation methods, a number of metallic selective chemistries have been developed, including diazonium (Strano et al., Science 301:1519-1522 (2003); Usrey et al., J. Am. Chem. Soc. 127:16129-16135 (2005)) and divalent chemistries such as [2+2] cycloadditions (Kanungo et al., Science 323:234-237 (2009)) and carbene cycloadditions (Kamaras et al., Science 301:1501 (2003)) which have been shown to effectively react with metallic CNTs prior to semiconductors. It is hypothesized that this metallic selectivity occurs as a result of the increased electron demand for these reagents. For instance, diazonium salts appear to undergo a charge-transfer complex before covalent addition with CNTs and the positively charged transition state is stabilized by the higher energy-level electrons at the Fermi level of metallic CNTs (Usrey et al., J. Am. Chem. Soc. 127:16129-16135 (2005)). However, these highly selective chemistries fail to provide a means to physically separate the functionalized metallic CNTs from semiconductor counterparts. The selectivity is typically lost long before enough functional groups can be coupled to the nanotubes to effect separation.